1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a separator in a horizontal stacking direction. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. The separator has a rectangular shape including long sides extending in the direction of gravity and short sides extending horizontally in a direction perpendicular to the stacking direction. The separator has a reactant gas flow field for supplying one of reactant gases along an electrode surface in the direction of gravity.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In practical use of such a fuel cell, normally, a predetermined numbers of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field for supplying a fuel gas to the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode are formed in the surfaces of the separator. Further, a coolant flow field as a passage of a coolant is formed between the separators along the surfaces of the separators.
In general, the fuel cell has internal manifold structure in which fluid supply passages and fluid discharge passages extending through the separators in the stacking direction are provided in the fuel cell. The fuel gas, the oxygen-containing gas, and the coolant as fluids are supplied to the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field through the respective fluid supply passages, and then, discharged into the fluid discharge passages.
As the fuel cell having the internal manifold structure, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 6-20713 is known. As shown in FIG. 8, the fuel cell includes a cell 1. The cell 1 has an air electrode 1b, a fuel electrode 1c, and a solid polymer membrane 1a interposed between the air electrode 1b and the fuel electrode 1c. Packings 2 are attached on both left and right sides of the cell 1. Main surfaces of the cell 1 and the packings 2 are sandwiched between a pair of separators 3. The separators 3 have fuel gas supply grooves 4 on a surface facing the fuel electrode 1c, and have oxygen-containing gas supply grooves 5 on a surface facing the air electrode 1b. The fuel gas supply grooves 4 and the oxygen-containing gas supply grooves 5 extend vertically.
A fuel gas supply passage 6a and an oxygen-containing gas supply passage 7a are provided at positions near upper corners of the separator 3, and a fuel gas discharge passage 6b and an oxygen-containing gas discharge passage 7b are provided at positions near lower corners of the separator 3.
The fuel gas supplied from the fuel gas supply passage 6a flows through the fuel gas supply grooves 4 downwardly in the direction of gravity, and the oxygen-containing gas supplied from the oxygen-containing gas supply passage 7a flows though the oxygen-containing gas supply grooves 5 downwardly in the direction of gravity.
However, in the conventional technique, the position of the fuel gas discharge passage 6b is above the lower end of the fuel gas supply grooves 4, and the position of the oxygen-containing gas discharge passage 7b is above the lower end of the oxygen-containing gas supply grooves 5. Therefore, water produced in the power generation reaction tends to be retained at the lower ends of the fuel gas supply grooves 4 and the oxygen-containing gas supply grooves 5. In particular, when operation of the fuel cell is stopped, and the fuel cell is exposed to the atmosphere at the temperature below the freezing point, the retained water freezes. Thus, due to expansion of the retained water, the fuel cell may be damaged undesirably.